The present invention is directed, in general, to power electronics and, more specifically, to a bias supply selection circuit, a method of operating a bias supply and a switching power supply employing the same.
The electronics industry continues to develop smaller equipment that performs many functions, which in turn may require increasingly more power from smaller, more compact power supplies. Additionally, such power supplies may have internal bias supply requirements for additional voltages that are different from the main power supply voltages. Bias supplies are used in power supplies to provide operating voltages needed by internal power control circuits or other circuits that provide proper operation during start-up or sustained power supply operation. The voltage and power requirements typically satisfied by bias supplies add another level of complexity to an already demanding environment.
Presently, there are several options for deriving a bias supply voltage such as that needed for control circuits that may be used in a small power supply unit. Typically, bias supply voltages are of approximately 12 volts with acceptable tolerances in the neighborhood of ten percent. Staying within this tolerance range requires more than an unregulated approach, especially when an input voltage to the power supply varies outside of its typical range.
The operating and safety demands of power supplies also add to the existing level of complexity. For example, a power supply should reliably ramp up under a start-up condition such as power-on. Additionally, the power supply typically should provide the ability to sustain a fault condition, such as a lower-than-rated impedance across its output. This is normally accomplished by limiting the output load current at an output voltage that is maintained near zero volts. This low output voltage fault condition may disrupt the switching bias supply associated with the power supply thereby impeding fault recovery when the fault is cleared.
One way of generating a bias voltage is to use separate magnetics to develop the bias voltage from an independent switching bias supply. This approach is, of course, usually more costly. Another approach uses a voltage developed in a winding coupled to an output inductor. This voltage may then be regulated further with a dissipative regulator. This particular approach suffers from the need to adequately isolate conductors on the primary and secondary sides of a boundary of a power transformer. Another common approach uses a linear regulating device, coupled to a source of input voltage of the power supply, to generate a bias supply voltage. Although this arrangement is reliable and predictable, linear regulating devices are usually highly dissipative regulators and when improperly controlled (e.g., controlled to be continually conducting) can reduce the overall efficiency of the power supply.
Accordingly, what is needed in the art is a way to provide a bias supply voltage that is predictable under, for instance, power supply start-up conditions or fault conditions and also provides improved overall efficiency.
To address the above-discussed deficiencies of the prior art, the present invention provides a bias supply selection circuit employable with a linear regulating device configured to provide a first bias supply voltage and a main bias supply stage configured to provide a second bias supply voltage. In one embodiment, the bias supply selection circuit includes a voltage monitoring circuit configured to monitor a level of the second bias supply voltage and to provide a control voltage proportional to the level. Additionally, the bias supply selection circuit includes a controllable switching circuit coupled to the voltage monitoring circuit and configured to select the second bias supply voltage by deactivating the linear regulating device when the control voltage reaches a predetermined value.
In another aspect, the present invention provides a method of operating a bias supply for use with a linear regulating device that provides a first bias supply voltage and a main bias supply stage that provides a second bias supply voltage. The method includes monitoring a level of the second bias supply voltage and providing a control voltage proportional to the level. The method also includes selecting the second bias supply voltage by deactivating the linear regulating device when the control voltage reaches a predetermined value.
The present invention also provides, in yet another aspect, a switching power supply including a primary power supply circuit having a main switch and coupled to a transformer. The switching power supply also employs a secondary power supply and bias supply circuit coupled to the transformer. The bias supply circuit employs a linear regulating device that provides a first bias supply voltage and a main bias supply stage that provides a second bias supply voltage. Additionally, the switching power supply also employs a bias supply selection circuit coupled to the bias supply circuit having a voltage monitoring circuit that monitors a level of the second bias supply voltage and provides a control voltage proportional to the level. The bias supply selection circuit also has a controllable switching circuit, coupled to the voltage monitoring circuit, that selects the second bias supply voltage by deactivating the linear regulating device when the control voltage reaches a predetermined value.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.